1. Field of the Invention
This invention relates to adaptive persistence processing in an ultrasound imaging system.
2. Description of Related Art
Ultrasound imaging systems generally operate by transmitting ultrasound signals from an ultrasound transducer or a plurality of transducer elements into a human body at a skin surface or within a body cavity, and receiving ultrasound signals reflected by objects or structures, such as organ tissue or other acoustic interfaces in a scan region (such as a scan plane), back to the ultrasound transducer.
Ultrasound images, like other images, are subject to noise which may affect the values of individual pixels. Ultrasound images, like some other images, display the effects of movement of the underlying object or objects being imaged. Ultrasound images, like some other images, suffer from the effects of temporal noise in the real-time image sequence. Simple filtering of this noise often has the untoward effect of blurring or degrading small structures, or the borders of structures, which are moving in the image field.
One method of the prior art is to use a single tap recursive filter which weights and sums corresponding pixel data from the previous display frame with pixel data from an input frame to generate each display frame; this is sometimes called "temporal filtering." In this method, a previous display frame's pixel would be averaged with an input frame's pixel, using a weighting value .alpha.. The weighting value .alpha. is constant, thus applying an equal degree of temporal filtering to all pixels in the frame. Thus, this method is data independent, i.e., not adaptive to changes in the image data. While this method achieves the goal of temporal noise reduction, it has the drawback that it can blur those structures or borders of structures which are moving. More generally, this method comprises a persistence filter in which the amount of persistence applied is the same for all pixels in the image, whether those pixels represent fast-moving structures, slow-moving structures, or low-intensity (i.e., dimmer) structures.
U.S. Pat. No. 4,751,846, "Reducing Noise in Ultrasonic Images", issued Jun. 21, 1988, in the name of Bruno Dousse, and assigned to Kontron Holding, A. G., relates to time-discrete non-linear filtering with filter coefficients which vary as a function of the filter input signal. This patent shows a two-stage nonlinear temporal filter which varies the coefficients for each stage as a function of the amplitude of the filter input signal. The temporal filter adjusts the coefficients of the two stages to apply a larger degree of filtering for smaller amplitude (i.e., dimmer) input signals and a smaller degree of filtering for larger amplitude (i.e., brighter) input signals. Thus, it is responsive only to the amplitude of the image signal. While this method also achieves the goal of temporally filtering pixels, it has the drawback that it does not successfully discriminate between moving structures and noise, and can result in increased noise levels in brighter structures, while blurring dimmer structures.
U.S. Pat. No. 4,785,818, "Method and Means or Dynamically Weighted Temporal Averaging of Pixel Data", issued Nov. 22, 1988, in the name of William R. Hardin, and assigned to General Electric Company, relates to a filter responsive to the difference between successive values of a pixel. This patent shows a nonlinear recursive filter with a weighting factor .alpha., having only a single stage. In this patent, the weighting factor .alpha. varies as a function of the first difference between the current pixel value and the corresponding pixel value from the just-previous frame. The function is such that the greater the difference, the less persistence is applied, so that fast-moving structures and noise are persisted less than slow-moving structures. While this method also achieves the goal of temporally filtering pixels, it has the drawback that this filter topology can often force the designer to choose between (1) attenuation of a broad range of higher frequencies, resulting in blurring moving structures, and (2) reduced noise attenuation.
U.S. Pat. No. 4,887,306, "Adaptive Temporal Filter for Ultrasound Imaging System", issued Dec. 12, 1989, in the name of Juin-Jet Hwang, et al., and assigned to Advanced Technology Laboratories, relates to filtering a pixel series with a filter whose characteristic varies as the magnitude of the pixel signal varies. This patent shows a nonrecursive filter structure, in which the weighting value .alpha. is adjusted as a function of the amplitude of the current pixel value. The function is such that the greater the amplitude (i.e., the brighter the pixel), the less persistence is applied, based on a proffered concept that brighter echoes are from "strong specular reflectors," thus preferably not enhanced by persistence, while dimmer echoes are from "diffuse scatterers," and thus more favorably enhanced by persistence. While this method also achieves the goal of temporally filtering pixels, it has drawbacks similar to that of the method shown in U.S. Pat. Nos. 4,751,846 and 4,785,818.